The present invention relates to a voltage-controlled circuit by which an electric characteristic (e.g., the gain of an amplifier, the attenuation degree of an attenuator or the frequency of an oscillator) is electronically changed in accordance with a control voltage and, in particular, to a voltage-controlled amplifier (or voltage-controlled attenuator) used for adjusting sound volume or any other analog quantity.
Conventionally, various electric characteristics, such as a gain, frequency, etc., have to be changed by a control voltage. In a sound volume control circuit adapted to audio/visual (A/V) equipment, the gain of an audio amplifier is changed with the voltage of a volume control signal. Such a sound volume control circuit is often made of circuit-integrated differential amplifiers controlled by a DC control voltage, and it can be embodied by, e.g., a gain control circuit of U.S. Pat. No. 4,065,725 (Lillis et al.) issued on Dec. 27, 1977.
A typical characteristic of the attenuation degree vs. control voltage of the above prior art circuit may be represented by curve C shown in FIG. 2A or 2B of this patent application (cf. FIG. 2 of U.S. Pat. No. 4,065,725). This characteristic teaches that below 1 volt of control voltage Vc (or above 3 volts of V.sub.EGC in FIG. 2 of the U.S. Pat. No. 4,065,725), the linearlity of curve C is degraded. In other words, within a range of high-attenuation degree (below -30 dB in FIG. 2B), the rate of change of a sound volume with respect to the change of control voltage Vc is very small. Consequently, under certain circumstances where a small sound volume is required to the operation of AV equipment, unless the set position of a sound volume controller is significantly changed, substantial changes for the sound volume cannot be achieved. Further, when a full mute of the sound volume by the manipulation of the controller is needed, it is practically impossible if the sound volume control circuit has a characteristic such as indicated by curve C in FIG. 2B because the maximum attenuation degree of curve C is about -35 dB. These are the disadvantages of the prior art. Actually, the linear range of the attenuation degree vs. control voltage should be as wide as possible, and the maximum attenuation degree should be as high as possible.